The invention relates to a method for operating an exhaust gas system, in particular of a vehicle, in which an amount of a reducing agent to be supplied to the exhaust gas of an engine is determined. To determine this amount measurements are evaluated, which indicate a content of nitrogen oxides in the exhaust gas downstream of a catalytic device. The catalytic device is adapted to diminish the content of nitrogen oxides in the exhaust gas. In determining the amount of the reducing agent be supplied to the exhaust gas, a magnitude and a frequency of the measurements are taken into account. Furthermore the invention relates to a control assembly for operating an exhaust gas system.
Today's vehicle engines, in particular diesel engines, are generally equipped with an aftertreatment system to meet stringent tail pipe emissions and on board diagnosis (OBD) regulations. The aftertreatment system or exhaust gas system usually includes a catalytic device such as a selective catalytic reduction (SCR) catalyst which is adapted to reduce the engine emitted pollutant NOx. In such an SCR catalyst nitrogen oxides react with a reducing medium in the form of ammonia (NH3) in a selective catalytic reduction reaction. The product of the reaction is nitrogen and water. The supply of ammonia into the exhaust gas can be achieved by injecting a so called diesel exhaust fluid (DEF) as a reducing agent, which is a urea-water solution with 32.5% urea. The injected diesel exhaust fluid then releases ammonia through a hydrolysis reaction after being mixed with the hot exhaust gas. Another way to supply the reducing agent to the exhaust gas is to inject ammonia in the gaseous phase directly into the exhaust gas.
Ammonia molecules are first adsorbed or stored on the surface of the SCR catalyst's coating which may consist of zeolite. The ammonia than reacts with NOx on the surface and converts to non-hazardous forms (N2 and H2O). The amount of ammonia supplied to the SCR plays a dominant role in the catalyst's NOx conversion efficiency. The optimal amount of ammonia supplied should match not only inflowing NOx, but also maintain a certain ammonia storage level, so as to result in the highest NOx conversion efficiency.
The amount of supplied ammonia can, however, deviate from the optimal amount due to various reasons. Some of the major reasons causing insufficient ammonia supply include the dilution of the diesel exhaust fluid with water, a certain degree of blockage of a dosing unit adapted to inject the diesel exhaust fluid into the exhaust gas and erroneously low sensor readings of the inflowing NOx, i.e., a false detection of the nitrogen oxide content upstream of the catalytic device. If ammonia is under-supplied, the SCR becomes less effective and the tail pipe NOx emissions will increase.
With tightened regulations regarding NOx emissions and on-board diagnosis issues, maintaining an optimal ammonia supply is critically important to assure the compliance of both emissions and OBD.
One way to obtain the amount of ammonia to be supplied to the exhaust gas is to use an ammonia sensor. However, the introduction of an additional sensor will not only increase the cost of the exhaust gas system and provide for additional OBD requirements, but also additional part failure can occur. The warranty risks associated with such part failure are also to be considered.
Using an ammonia sensor provides for collectively detecting an insufficient reducing agent supply level which is caused by multiple deviation sources. However, as mentioned above the drawbacks to this are added hardware costs, added OBD requirements, added part failure modes and increased warranty related cost risks.
Document US 2011/0005202 A1 describes an exhaust gas system with a first sensor located upstream of an SCR catalyst and a second sensor located downstream of the SCR catalyst. The frequencies and the magnitudes of the signals or measurements provided by the NOx sensor located downstream of the SCR catalyst are compared to the frequencies and magnitudes of the signals provided by the upstream sensor. Based on a comparison of the frequencies and magnitudes the ammonia storage level of the SCR catalyst is determined. Thus, the conversion efficiency of the SCR catalyst is monitored.
Document WO 2008/048175 A1 describes a method for monitoring the functioning of an exhaust gas aftertreatment system of a motor vehicle based on a measuring signal from a sensor located downstream of a catalyst. The signal provided by the sensor represents the NOx content in the exhaust gases flowing out of the catalyst. A frequency analysis of the measuring signal provided by the sensor is performed during a certain period of time. Thus, an evaluation value is established, which reflects the character of a frequency part of the measuring signal provided by the sensor during this period of time. Information regarding the functioning of the exhaust gas aftertreatment system is generated based on this evaluation value.
However these methods are prone to false determinations of the amount of reducing agent to be supplied to the exhaust gas under some circumstances.
It is therefore an object of the present invention to provide a method and a control assembly of the initially mentioned kind, which is particularly reliable in determining the amount of the reducing agent to be supplied to the exhaust gas.
In the method according to the invention a plurality of measurements are captured during a predetermined period of time. A magnitude of a measurement captured within this predetermined period of time is related to a quantity which is derived from the respective magnitudes of the plurality of measurements captured during the predetermined period of time. The related measurement is then utilized to determine the amount of the reducing agent to be supplied to the exhaust gas. As the measurement is related to the quantity, a relative magnitude-frequency analysis is utilized instead of an absolute magnitude-frequency analysis. Thus, a particularly high detection resolution is achieved with either a new or fresh catalytic device or an aged catalytic device.
The method provides a real time on-board diagnostic detection tool without a physical ammonia sensor to detect if the ammonia or such a reducing agent has been insufficiently supplied. Thus, the dosing amount can be adjusted accordingly. Therefore insufficient supply of the reducing agent to the catalytic device can be detected by utilizing the existing tail pipe NOx sensor signal or measurement.
One of the major advantages of this method is its capability to collectively detect an ammonia supply insufficiency caused by multiple small deviations from various sources which each alone may not be detectable. This is based on the finding that for an individual detection of deviation sources a dilution of the reducing agent would need to be detected as well as a too high or too low delivery quantity of the reducing agent or a nitrogen oxide sensor reading drift which is high or low. If it is intended to detect such individual deviation sources the drawback is that this may not give an accurate total effect of the reducing agent supply level when adding up all the deviation sources detected. This might be the case if the detection resolution is poor at one of the deviation source detection sites.
For example, the detection of a NOx drift in the exhaust gas leaving the engine has today a poor resolution or accuracy. This low resolution is a hurdle in making a reducing agent control compensation adjustment. As the present method allows to detect the individual deviation sources collectively, the detection resolution is increased. Thus, the method is particularly reliable in determining the amount of the reducing agent to be supplied to the exhaust gas.
The method provides a solution to control the amount of reducing agent actually supplied to a catalytic device such as an SCR catalyst to be maintained always at an optimal level even under various hardware related deficiency and malfunction conditions. By detecting an insufficiency in the amount of the reducing agent supplied to the exhaust gas on-board and by adjusting the dosing control, actual dosing errors caused by such deficiencies and malfunctions can be compensated for. Thus, a particularly low tail pipe nitrogen oxide emission level can be achieved and maintained, and OBD fault occurrences can be avoided. Furthermore the costs for using a physical sensor adapted to detect the amount of the reducing agent supplied to the exhaust gas can be avoided.
A particularly easy way to take into account the respective magnitudes of the plurality of measurements captured during the predetermined period of time is to utilize an average or a median of the magnitudes of the plurality of measurements. Thus, a variation of the absolute magnitude of the measurements does not have an unwanted influence on the determination of the amount of the reducing agent being supplied to the exhaust gas. For example, an aged catalytic device with a sufficient supply of reducing agent will not be mistakenly detected as an insufficient supply of the reducing agent.
In a further advantageous embodiment a moving median is utilized as the quantity. The moving median takes into account a series of predetermined periods of time. In a particularly simple configuration the moving median is a mathematic average of the magnitudes of the plurality of measurements which are symmetrically arranged around the measurement or value to be related to the moving median. Thus, the influence of an overall trend of the measurement magnitudes can be detected and is not falsely interpreted as an inappropriate amount of the reducing agent supplied to the exhaust gas.
Thus, utilizing the moving median as the quantity further enhances the reliability of the determination of this amount.
It has further proven to be advantageous if in relating the measurement to the quantity a difference between the magnitude of the measurement to be related and the quantity is calculated. By taking into consideration this difference, the noise of the measurements or signals is detected. This is based on the finding that the quantity of the reducing agent stored in the catalytic device has a damping effect on sudden variations in inflowing nitrogen oxides, in particular on sudden increases of inflowing nitrogen oxides. If the level of reducing agent stored in the catalytic device is reduced or if there is no stored reducing agent left at all, the reduced damping capacity to the inflowing nitrogen oxide variations results in the noisiness of the tail pipe nitrogen oxide sensor's signal. Therefore, by taking into account the noisiness a degree of shortage in the amount of the reducing agent supplied to the catalytic device can be readily detected.
Further advantageously a total of absolute values of differences is created in utilizing the related measurement to determine the amount of the reducing agent to be supplied to the exhaust gas. Such a total of absolute values is particularly easy to handle and has proven to be a very robust detection tool.
Herein, it has appeared advantageous if for the total a lower threshold and an upper threshold are defined. Then the amount of the reducing agent to be supplied to the exhaust gas is varied if the total is below the lower threshold or above the upper threshold. Thus, a closed-loop control to adjust the amount of the reducing agent supplied to the exhaust gas can be particularly easily implemented.
The adjustments made to the dosing of the reducing agent can in particular be performed by a PID controller (proportional-integral-derivative controller). This provides for a particularly cheap and flexible implementation.
Preferably the lower threshold is above zero. Thus, an overdosing scenario can be avoided. This is based on the finding that with a lower threshold of zero or very near to zero it is difficult to tell whether the reducing agent is oversupplied or not.
It has therefore proven to be advantageous, if as the lower threshold a value between 0.1 and 1, in particular between 0.3 and 0.8 and as the upper threshold a value between 0.9 and 1.5, in particular between 1 and 1.4 are defined. Such values have shown very good capability in responding to a change in the supply of the reducing agent and in maintaining the right supply level of the reducing agent. This is in particular true, if the lower threshold value is between 0.4 and 0.6 and the upper threshold value between 1.2 and 1.3.
A particularly safe determination of the amount of the reducing agent to be supplied to the exhaust gas is achieved, if the total of absolute values of differences is created over a sampling period which is a multiple of the predetermined period of time. The sampling period can therefore in particular be in the range of 2 minutes to 20 minutes, preferably in the range of 5 minutes to 15 minutes. A particularly good result in maintaining a correct level of reducing agent supply is achieved, if the sampling period is about 10 minutes.
Finally it is proven advantageous if the predetermined period of time is in the range of 5 seconds to 60 seconds, in particular in the range of 10 seconds to 30 seconds. This is based on the finding that a period of time which is too short may result in an inability to reliably detect an insufficiency in the dosing of the reducing agent. If, however, the predetermined period of time is too long, the sensitivity for noise detection is decreased. Therefore, the length of the predetermined period of time can in particular be about 20 seconds.
The control assembly according to the invention for operating an exhaust gas system includes a dosing unit which is adapted to supply an amount of a reducing agent to the exhaust gas of an engine. The control assembly further includes an evaluation unit which is adapted to evaluate measurements which indicate a content of nitrogen oxides in the exhaust gas downstream of a catalytic device of the exhaust gas system. The catalytic device is adapted to diminish the content of nitrogen oxides in the exhaust gas. The evaluation unit is adapted to take into account a magnitude and a frequency of the measurements in determining the amount of the reducing agent to be supplied by the dosing unit. Herein the evaluation unit is adapted to capture a plurality of measurements during a predetermined period of time, to relate a magnitude of a measurement captured within this period of time to a quantity which is derived from the respective magnitudes of the plurality of measurements captured during the predetermined period of time. The evaluation unit is further adapted to utilize the related measurement in order to determine the amount of the reducing agent to be supplied to the exhaust gas by the dosing unit.
Such a control assembly provides for a particularly reliable determination of the amount of the reducing agent which shall be introduced into the exhaust gas by the dosing unit.
The advantages and preferred embodiments described for the method according to the invention also apply to the control assembly according to the invention and vice versa.
The features and feature combinations mentioned above in the description as well as the features and feature combinations mentioned below in the description of figures and/or shown in the figures alone are usable not only in the respectively specified combination but also in other combinations or alone without departing from the scope of the invention. Thus, implementations not explicitly shown in the figures or explained, but which result and can be generated by separated feature combinations of the explained implementations are also to be considered encompassed and disclosed by the invention.
Further advantages, features and details of the invention are apparent from the following description of preferred embodiments as well as based on the drawings.